1. Field of the Invention
This invention relates to a packaging bag for seal-packaging of food, etc. Particularly, this invention relates to a packaging bag which can be heated in its sealed state for retaining food, etc., therein, with an electromagnetic wave heating system such as a microwave oven.
2. Related Art
In order to eat the food which is seal-packaged into the packaging bag or the like, there may be cases where heating or heat-cooking by the microwave oven is appreciated or needed. In such cases, the internal pressure of the packaging system will be built up with the steam generated by the heating as long as the packaging is kept as tight seal condition. As the result, the packaging bag explodes, and contents in the bag are scattered over inside of microwave oven. Such problem is well known in the art and various methods for solving the problem have been proposed.
For example, in Japanese Utility Model Registration No. 3048391, a packaging bag which has a notch at side seal portion thereof has been proposed. In JP-H10-72070A, a packaging bag which has a notch capable of breaking the seal condition of the bag has been also proposed. In this literature, the notch is formed at a wing-like part which is branched from a main body of the packaging bag and has an inner space communicated with the inner space of main body. In JP-2000-72187A, a packaging bag which has a prearranging portion for steam release port being arranged from a breakable notch and in a line with another prearranging portion for contents pouring port has been proposed. In JP-2000-118575A, it is disclosed that a thin film tape of easy peel-off type is used for a part of opening of self-standing pouch and that the seal by the tape part concerned is released by the internal pressure of pouch. In JP-2000-168851A, a weak seal part is formed in a part of seal portion at a bottom of bag, and the weak seal part concerned is released by the internal pressure of the bag. In JP-10-95471A, the packaging bag which has a weak seal area formed in a part of edge seal part, and further has a narrow seal part formed in a part of the weak seal area is proposed. In the packaging bag of JP-9-142541A, while making a central seal part into a weak seal part by carrying out heat fusion at low temperature rather than other parts, two or more linear gap parts is formed in the bottom seal part. In the packaging bag of JP-9-240754A, a part of the bag is made of nonwoven fabric in order to release the steam therefrom. In the container for microwave oven heating of JP-H2-84908A, a V-shape seal portion which protrudes interiorly is prepared as an area of edge seal part of the container, so as to concentrate the internal pressure of the container at the portion.
However, in the case of Japanese Utility Model Registration No. 3048391, when the food to be stored therein is that possesses fluidity, such as liquid, etc., such a bag cannot be adopted since the contents can easily leak out. Moreover, in JP-H10-72070A or JP-2000-72187A, the work of making a cut or hole in advance of heating is troublesome. Furthermore, to forget the work before heating is liable to occur. In that case, the bag will explode, and it is so dangerous. Since the types of easy peel-off and weak seal disclosed as JP-2000-118575A, JP-2000-168851A, and JP-10-95471A express insufficient seal strength, such bags have a possibility of seal-breakage during transportation. In JP-9-240754A, the sealing performance as a packaging bag can not be expected.